Idle Speed: Testing and Inspection

Fig. 4 Vapor Canister Hose Restrictor:

***UPDATED BY TSB 258706 June 1987

Adjustment of the idle air stabilizer system is limited to measuring the duty cycle of the valve and adjusting it to specification with the air bypass screw in the throttle housing. The duty cycle is the measurement of the ON time of the cycling ON/OFF voltage signal to the valve, and is expressed in percent. This percentage of ON time is measured by connecting a duty cycle meter to the test connector in the wiring harness near the ignition coil.
A common dwell meter can also be used to measure the duty cycle by using the 90° scale. The 90° scale corresponds to the 100% scale of a duty cycle meter. Example: a duty cycle of 50% is the same as a dwell of 45°.
1. Before checking the idle duty cycle:
a. Disconnect the crankcase ventilation hoses and vent them to atmosphere.
b. Remove the charcoal canister line "T" fitting from the air flow sensor boot and turn it to insert the side with the 1.5 mm (0.059 in) restrictor hole in the boot, as shown in Fig. 4. If the car is not equipped with a restrictor type "T" fitting, a plug with the proper hole size is available from Volkswagen.
2. With the engine running and at normal operating temperature (radiator fan should cycle at least once), duty cycle should be 26 - 30% (23° - 27° dwell).
3. If the duty cycle is not within specifications, adjust it by turning the air bypass screw in the throttle housing.
4. Possible problems:
a. If the idle does not adjust, check the idle switch for function, and the wiring to the control unit for open or short circuits between the switch and the injection control unit.
b. If the duty cycle is 33% or 40% recheck that the A/C is turned OFF. Check the idle switch for function, and check wiring for open or short circuits between the switch and the injection control unit.
c. If the duty cycle fluctuates irregularly between 20% - 25%, check for an open rpm signal circuit between the ignition control unit and the injection control unit.
d. If the duty cycle is a steady 25%, the idle speed is incorrect. Check for leaks at the throttle housing gaskets, and inspect all seals and hoses connected with the intake system. Also check for an open circuit between the idle control valve and the injection control unit.